The present invention relates generally to a method and improved devices for increasing the production of oil. More specifically, the bottomhole tool and the method of the invention provide for maintaining the bottomhole pressure at a level considered optimum for maximizing oil production in a well with high gas-to-oil ratio (GOR). The most advantageous implementation of the present invention is in wells with high GOR defined as GOR greater than 600 cubic feet per barrel. In these wells the tool and the method of the invention can be used when the bottomhole pressure is lower than the bubble point pressure as well as in all cases when the gas cone has appeared such as in flowing, gas lift, and pump regimes of oil production. Another useful implementation of the invention is in GOR wells when a so-called gas cone or gas skin effects take place. These detrimental effects generally lead to destabilization of the well production process, fast increase of GOR and difficulties in managing the well.
This invention contains further improvements of my earlier U.S. Pat. No. 7,172,020 entitled “Oil Production Optimization and Enhanced Recovery Method and Apparatus for Oil Fields with High Gas-To-Oil Ratio”, incorporated herein in its entirety by reference.
Optimization of oil production has been a goal of many methods and devices of the prior art. Generally speaking, the bottomhole behavior of oil mixed with gas and some other ingredients such as water, etc. has been described in a series of mathematical equations by Muskat. One specific publication of Muskat is incorporated herein by reference in its entirety and describes the mathematical model of oil reservoir: Muskat M. “The Production Histories of Oil Producing Gas-Drive Reservoirs”, published in the Journal of Applied Physics in March of 1945, p.147-159.
For illustration purposes, a one-dimensional axis-symmetrical system of Muskat equations with corresponding PVT characteristics of fluid and dependencies of relative permeability Kro, Krg from liquid saturation (So) can be described as follows:
                    1        r            ⁢              ∂                  ∂          r                    ⁢              (                  r          ⁢                                    k              ro                                                      μ                o                            ⁢                              B                o                                              ⁢                                    ∂              p                                      ∂              r                                      )              =                  -        158.064            ⁢              ϕ        k            ⁢              ∂                  ∂          t                    ⁢              (                              S            o                                B            o                          )                                1        r            ⁢                        ∂                      ∂            r                          ⁡                  [                                    r              ⁡                              (                                                                            k                      rg                                                                                      μ                        g                                            ⁢                                              B                        g                                                                              +                                                            Rs                      5.615                                        ⁢                                                                  k                        ro                                                                                              μ                          o                                                ⁢                                                  B                          o                                                                                                                    )                                      ⁢                                          ∂                p                                            ∂                r                                              ]                      =                  -        158.064            ⁢              ϕ        k            ⁢              ∂                  ∂          t                    ⁢              (                                            S              g                                      B              g                                +                                                    S                o                                            B                o                                      ⁢                          Rs              5.615                                      )            where: P—pressure in formation; So—oil saturation in formation; Sg—gas saturation in formation; Rs—solution of gas in oil; Bo—oil formation volume factor; Bg—gas formation volume factor; μo—oil viscosity; μg—gas viscosity; φ—formation porosity; K—formation permeability.
For practical purposes, Vogel had simplified the Muskat equations and adapted them to the calculations of oil producing formations. These equations are known as Vogel model and have subsequently been modified by others. One example of such publication is as follows: Vogel, Inflow Performance Relationships for Solution-Gas Drive Wells, as published in Journal of Petroleum Technology, January 1968, pp. 83-92, incorporated herein in its entirety by reference. Unfortunately, Vogel model does not work well in wells with high gas-to-oil ratio. According to Vogel, the dependency of oil rate production of bottomhole pressure is a constantly diminishing parabolic curve with a production peak at zero value of the bottomhole pressure, see for example FIG. 2 of the above mentioned article. In other words, the lower the bottomhole pressure, the higher the oil rate production from the formation. This is a gross simplification of the bottomhole processes in the formation. In fact, if the bottomhole pressure falls below saturation pressure in case of high GOR, relative permeability coefficient by oil decreases because of gas saturation increase, which in turn is a result of gas being released from oil. Viscosity of so degassed oil also increases. This leads to a decrease of productivity index of formation. This phenomenon affects the oil production rate more than the increasing depression. As a result, decreasing of the bottomhole pressure below saturation pressure can lead to a decrease in oil production rate, rather than to its increase as predicted by Vogel's model, see FIG. 1. In some extreme cases, reliance on Vogel's model will cause a complete switch in production from oil to gas. There is a need therefore for a method allowing calculating the oil production rate in high GOR wells with better accuracy then that allowed by Vogel's model.
It is also known that producing oil wells with high GOR (Gas-to-Oil Ratio) often lose their stability, and this process is accompanied by a sharp increase in GOR. Any attempts to stop this process by using a surface choke or other surface manipulations usually fail, and the well gradually switches into a gas mode. The physics of this process can be explained as follows: in case when a gas cone covers some holes of a perforated section of the well, quite often that well loses stability. This, in turn, leads to a continuing slow increase of the cone height followed by an increase in the gas stream and a decrease in the oil flow. This process continues until the well is completely switched to a gas mode. Even if a switch to a gas mode does not happen, the instability of the well does not allow efficient control of the bottomhole pressure by using a choke at the surface. Similar detrimental phenomena can occur because of formation of a gas skin effect near the bottom of the well. The physics of the skin effect is described in detail in my '020 patent. It also shows that this phenomena leads to a non-conventional shape of the IPR curve (Inflow Pressure Relationship, i.e. the dependence of well oil flow rate of the bottomhole pressure). A notable feature of this curve is the presence of a certain threshold value of the bottomhole pressure (called “Popt—optimal pressure”), at which the greatest possible oil flow rate from a reservoir can be achieved (FIG. 1).
The need exists therefore for a device and method of restoring and maintaining the stability of production in high GOR wells even in the presence of gas cone and gas skin effects.